diff --git a/requests/sessions.py b/requests/sessions.py
index 3f59cab9..0cdcf4fe 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -288,8 +288,20 @@ class SessionRedirectMixin(object):
         if 'Proxy-Authorization' in headers:
             del headers['Proxy-Authorization']
 
+        def sanitize_proxy_url(proxy_url):
+            # Remove leading zeros in the proxy URL's IPv4 address
+            parsed_url = urlparse(proxy_url)
+            if parsed_url.hostname:
+                hostname_parts = parsed_url.hostname.split('.')
+                if len(hostname_parts) == 4 and all(part.isdigit() for part in hostname_parts):
+                    sanitized_hostname = '.'.join(str(int(part)) for part in hostname_parts)
+                    sanitized_url = parsed_url._replace(netloc='{}:{}'.format(sanitized_hostname, parsed_url.port))
+                    return sanitized_url.geturl()
+            return proxy_url
+
         try:
             username, password = get_auth_from_url(new_proxies[scheme])
+            new_proxies[scheme] = sanitize_proxy_url(new_proxies[scheme])
         except KeyError:
             username, password = None, None
 
